Originally Posted by chucklesR
As long as you don't let it fill with water and therefore allow room to expand it won't break anything.
Leave the covers off and the bilge pump powered (provided you have a recharge method).
NO!!!!! I have seen far too many melted & blistered bilge pumps that nearly burned entire marinas down. Powering 12V into a stalled pump creates heat. Enough heat to melt the pump or potentially start a fire. Please do not leave a bilge pump connected to a 12V source if there is potential for freezing... This is but one of the many reasons most boat yards, and their insurers, stipulate that batteries must be disconnected while boats are on the hard
Best bet is to visit the boat regularly during warm spells and manually drain it.. Adding some PG wont hurt and may drop the slush/freeze point slightly, unless it gets to diluted by incoming fresh water.. A garboard drain is a good option if your bilge lends well to its installation..